1. Field of the Invention
The present invention relates to an electrostatic type tilt sensor which is used for detecting a tilt angle with respect to a plane perpendicular to the direction of gravity, more particularly to a tilt sensor providing an alarm or conducting a predetermined control when a detected tilt angle exceeds a predetermined value.
2. Background Art
Conventional tilt sensors employing a tilt detection element are described in Japanese Unexamined Utility Model Publication No. 4-53528 and Japanese Examined Utility Model Publication No. 5-14168. An electrostatic type tilt sensor having these types of conventional structure is shown in FIG. 5 and FIG. 6.
FIG. 5 is an exploded view of a conventional tilt sensor. FIG. 6 is a cross-sectional view of a conventional tilt sensor along a plane cut normal to a front surface of the tilt detection element.
A printed circuit board 1 made of a heat resistant material, for example a laminate plate made of glass cloth and epoxy resin, is disposed vertically with respect to a reference plane for measuring a tilt angle when a tilt sensor is fixed to an object whose tilt angle is to be measured. In FIG. 5, this reference plane is designated by the plane including an imaginary line L0 represented by a double-dotted and dashed line. This plane defined by the imaginary line L0 becomes a reference plane from which to measure tilt angle.
The reference plane is in a xe2x80x9c0 degreexe2x80x9d tilt angle when the reference plane includes a line normal to the direction of gravity. In the printed circuit board 1, a pair of differential electrodes 2a, 2b are formed of a copper foil pattern electrically independent of each other in two regions. The two regions (left and right) are defined by a plane formed along the intersection of (an imaginary line L1 shown by a double dot and dash line in FIG. 5) both the reference plane and the surface of the printed circuit board 1.
The signal processing circuit section of the tilt sensor, which includes a printed wiring pattern and related electronic parts will be described hereinafter. The signal processing circuit section is mounted on a surface opposite to the surface on which the differential electrodes 2a, 2b of the printed circuit board 1 are formed. The respective differential electrodes 2a, 2b are connected to the copper foil pattern on the surface of the printed circuit board 1. The signal processing circuit section is formed via through holes, at the electrode points 2c, 2d shown in FIG. 5.
The pair of differential electrodes 2a, 2b are formed as an electrode pattern which is symmetric with respect to the imaginary line L1. Also, each electrode of the pair of differential electrodes 2a, 2b is formed as an electrode pattern which is symmetric with respect to the imaginary line L2. Imaginary line L2 is the line that is normal to the imaginary L1 in FIG. 5. In the example shown in FIG. 5, each of the differential electrodes 2a, 2b is shaped like a horizontal fan.
In the example shown in FIG. 5, the arc-shaped periphery of each of the differential electrodes 2a, 2b follows the shape a circular arc. The circular arc is defined by a circle with its center at the point of intersection of the imaginary line L1 and the imaginary line L2. In this example, the diameter of the circle is set at 30 mm.
A reference numeral 3 designates a common electrode plate formed of a conductive material having a desired rigidity. As shown in FIG. 6, this common electrode plate 3 is mounted on the printed circuit board 1 in a state where it is held in parallel to the differential electrodes 2a, 2b with a certain gap between them. A plurality of terminals 3a, 3b, 3c, 3d are inserted into the printed circuit board I which are integral with the common electrode plate 3 and are formed by bending the plate 3 at right angles. The terminals 3a, 3b, 3c, 3d are inserted into terminal holes 4a, 4b, 4c, 4d made in the printed circuit board 1 and are secured by soldering them to the surface of the printed circuit board 1 on which the signal processing circuit section is formed.
An oil case 5 formed of plastics having a desired flexibility is formed in the shape of a letter U in cross section. When an end face of the oil case 5 is bonded to the printed circuit board 21 with bonding means such as a double-faced adhesive tape 5B or the like, the oil case 5 forms a closed space with the surface of the printed circuit board 1.
The peripheries of the differential electrodes 2a, 2b, the periphery of the common electrode 3, and the periphery of the case 5 are formed concentrically with each other. The opposite faces of the differential electrodes 2a, 2b, that of the common electrode 3, and the corresponding face of the case 5 are formed in parallel to each other.
The closed space formed by the case 5 and the printed circuit board 1 is filled with a dielectric liquid 7 such as a silicone oil or the like. The dielectric liquid 7 is poured from a through hole 6 made in the printed circuit board 1 to the level of approximately half the effective volume in the closed space, e.g. to the level of the imaginary line L2 shown in FIG. 5. The through hole 6 of the printed circuit board 1 is filled with the dielectric liquid 7 and is then sealed.
An electrostatic shielding plate 8 is mounted on a side of the printed circuit board 1 to cover the case 5 and its surroundings and the electrostatic shielding plate 9 is mounted on a second side of the printed circuit board 1 to cover the signal processing circuit section described hereinafter.
FIG. 7 is a schematic view of a signal processing circuit section of an exemplary, conventional tilt sensor. In FIG. 7, an oscillator 11 and the output terminal thereof are connected to the common electrode plate 3 of the tilt detection element 10 having the characteristics described in FIG. 5 and FIG. 6. Further, the pair of differential electrodes 2a, 2b of the tilt detection element 10 are connected to the input terminals of capacity-voltage conversion circuits 12a, 12b, respectively.
The output terminals of the capacity-voltage conversion circuits 12a, 12b are connected to the input terminals of a differential amplifier circuit 13, respectively. An output terminal 14 of the tilt sensor is led out of the differential amplifier circuit 13. The signal processing circuit section is provided with a power stabilizing circuit 15 and the stabilized voltage supplied from this power stabilizing circuit 15 is supplied to the oscillator 11 and the differential amplifier circuit 13 as a power supply voltage.
Since the signal processing circuit section is arranged in the manner described hereinabove, an oscillation output signal of a predetermined frequency from the oscillator 11 is supplied to the capacity-voltage conversion circuits 12a through a first capacitor connected by the differential electrode 2a and the common electrode plate 3 and also to the capacity-voltage conversion circuits 12b through a second capacitor connected by the differential electrode 2b and the common electrode plate 3.
Accordingly, peak value signals corresponding to the capacity of the first capacitor and the capacity of the second capacitor are applied to the capacity-voltage conversion circuits 12a, 12b, respectively.
The capacity-voltage conversion circuits 12a, 12b rectify the input signals, and produce smoothed voltage. Therefore, the respective output voltages of the capacity-voltage conversion circuits 12a, 12b correspond to peak values of the input signals. The capacity of the first capacitor and the capacity of the second capacitor correspond to their respective input signals.
Therefore, the differential amplifier circuit 13 produces a differential voltage between the output voltage of the capacity-voltage conversion circuits 12a and the output voltage of the capacity-voltage conversion circuits 12b as the output of the tilt sensor at the output terminal 14. In summary, the differential amplifier circuit 13 produces an output voltage corresponding to the difference in capacity between the first capacitor and the second capacitor.
The tilt sensor, provided with the tilt detection element 10 and the signal processing circuit section, is mounted on a plane to be measured, e.g. the reference plane for measuring the tilt of the object to be measured as described above. The tilt sensor is placed such that the surface of the printed circuit board 1 of the tilt detection element 10 becomes the plane to be measured.
When a plane to be measured is not tilted in the tilt direction that is desired to be measured, a differential voltage of zero is obtained. The plane to be measured includes a line normal to the direction of gravity and essentially remains unchanged. Under this condition, the dielectric liquid 7 is brought into a state where approximately half of the respective differential electrodes 2a, 2b are immersed dipped in the dielectric liquid 7. Accordingly, the capacity of the first capacitor associated with the differential electrode 2a and the common electrode plate 3 are equal to the capacity of the second capacitor associated with the differential electrode 2b and the common electrode plate 3. Therefore, the difference in output voltage between the capacity-voltage conversion circuits 12a, 12b becomes zero. The output voltage of the differential amplifier circuit 13 becomes a corresponding voltage Vo.
When the plane to be measured is tilted in the tilt direction to be measured, the liquid levels of the dielectric liquid 7 will change. The liquid level of the dielectric liquid 7 is brought into a state where one of the differential electrodes 2a, 2b is immersed in the dielectric liquid 7 in proportion to the tilt angle experienced by the plane being measured (e.g., surface of the printed circuit board 1) and the opposite differential electrode 2a, 2b is no longer exposed to the dielectric liquid 7 in proportion to the tilt angle experienced by the plane being measured. A difference in capacity is produced by this effect corresponding to (and representative of) the tilt angle between the first capacitor and the second capacitor.
As seen in FIG. 5, when the plane to be measured is tilted in the +xcex8 direction (for example, counterclockwise) from the position where the tilt angle is 0 degree, the capacity of the first capacitor is decreased and the capacity of the second capacitor is increased. Therefore, the output voltage of the capacity-voltage conversion circuit 12a becomes larger than that of the capacity-voltage conversion circuit 12b. Therefore, the output voltage of the differential amplifier circuit 13 is made larger than the voltage Vo by the amount corresponding to the tilt angle in the +xcex8 direction.
On the other hand, when the plane to be measured is tilted in the xe2x88x92xcex8 direction (for example, clockwise) from the position where the tilt angle is 0 degree, the capacity of the second capacitor is decreased and the capacity of the first capacitor is increased and hence the output voltage of the capacity-voltage conversion circuit 12a becomes smaller than that of the capacity-voltage conversion circuit 12b. Therefore, the output voltage of the differential amplifier circuit 13 is made smaller than the voltage Vo by the amount corresponding to the tilt angle in the xe2x88x92xcex8 direction.
The differential amplifier circuit 13 produces the voltage corresponding to the difference in the output voltage between the capacity-voltage conversion circuits 12a, 12b, or the difference in capacity between the two capacitors.
FIG. 8 is a graphical view showing output voltage characteristics of a conventional tilt sensor. As seen in FIG. 8, the output voltage of the differential amplifier circuit 13 is a direct current varying linearly in proportion to the tilt angle of the plane to be measured from the position where the plane to be measured at tilt angle=0, wherein the tilt angle includes the tilt direction from the position where the plane to be measured at tilt angle=0.
In aforementioned structure, the differential electrodes 2a, 2b and the common electrode plate 3 are shaped like the fan described above so that the output voltage of the differential amplifier circuit 13 corresponds linearly to a change in the tilt angle, as shown in FIG. 8.
The measurement of tilt angle through this type of conventional tilt sensor is often used to provide an alarm indicating that the plane to be measured is tilted more than a predetermined value. A threshold voltage is set for the output voltage of the differential amplifier circuit 13. An alarm is issued when the output voltage exceeds the threshold voltage. Here, since the output voltage characteristics of a conventional tilt sensor vary linearly with respect to the tilt angle, threshold voltages are set for each tilt angle direction, e.g. a threshold voltage V1 corresponding to a threshold tilt angle for tilt angles in the +xcex8 direction and a threshold voltage V2 corresponding to a threshold tilt angle for tilt angles in the xe2x88x92xcex8 direction.
An alarm circuit includes a first detection circuit for issuing an alarm when the output voltage of the differential amplifier circuit 13 is larger than the voltage V1 when the plane to be measured is tilted in the +xcex8 direction and at a second detection circuit for issuing an alarm when the output voltage of the differential amplifier circuit 13 is smaller than the voltage V2 when the plane to be measured is tilted in the xe2x88x92xcex8 direction.
However, in the aforementioned arrangements using a conventional tilt sensor, the alarm circuit is necessarily complex. Since the alarm circuit is required to set different threshold voltages and to have different detection circuits for both tilt directions (+xcex8 and xe2x88x92xcex8) from the position where the plane to be measured is at the tilt angle of 0 degrees.
The present invention overcomes the shortcomings associated with the related art and achieves other advantages not realized by the related art.
An aspect of the present invention is to provide a tilt sensor capable of detecting variations in tilt angle for use in an alarm circuit utilizing only one threshold value.
An aspect of the present invention is to provide an alarm circuit having a tilt sensor that utilizes one threshold value for tilt angle that is indicative of tilt angle, irrespective of the direction of tilt of the plane to be measured from the 0 degree or no-tilt position.
These and other aspects are accomplished by a tilt sensor comprising a printed circuit board disposed in a direction normal to a reference plane for measuring a tilt angle; a pair of differential electrodes mounted on the printed circuit board; a common electrode plate opposed to the pair of differential electrodes with a predetermined gap formed between the differential electrodes; a common electrode fixed to the printed circuit board by a terminal formed from the common electrode plate; a dielectric liquid filled into a closed space in a state where its surface level varies according to the tilt angle of the reference plane; a signal processing circuit section formed on the printed circuit board, the signal processing circuit section producing an output signal corresponding to a difference in capacity between two capacitors, each capacitor includes the common electrode and a respective differential electrode of the pair of differential electrodes, as a tilt angle detection output, wherein the pair of differential electrodes are mounted electrically independently of each other in upper and lower regions respectively, wherein the upper and lower regions are divided by a horizontal centerline, the horizontal centerline passing through a center of gravity of the printed circuit board in a direction normal to a direction of gravity when the printed circuit board is in a xe2x80x9cno tiltxe2x80x9d condition, and the pair of differential electrodes are formed in shapes symmetrical to said horizontal centerline and a vertical centerline of the printed circuit board, the vertical centerline passing through said center of gravity and normal to the horizontal centerline; and an oil case for storing the pair of differential electrodes and the common electrode plate and forming the closed space between itself and the printed circuit board.
These and other aspects are accomplished by a tilt sensor comprising a printed circuit board disposed in a direction normal to a reference plane for measuring a tilt; a pair of differential electrodes mounted on the printed circuit board, which are electrically independent of each other; a common electrode plate opposed to the pair of differential electrodes with a predetermined gap therebetween; a common electrode fixed to the printed circuit board by a terminal formed of the common electrode plate; a case body for storing the pair of differential electrodes and the common electrode plate in the closed space formed by itself and the printed circuit board; a dielectric liquid filled into the closed space in the state where its surface level varies according to the tilt of the reference plane; and a signal processing circuit section which is formed on the printed circuit board and is capable of producing the output signal of the level corresponding to the difference in capacity between two capacitors, each of which is constituted of the common electrode and each of the pair of the differential electrodes, as a tilt detection output, wherein the pair of differential electrodes are mounted electrically independently of each other in the regions which are divided by a first intersection where a plane parallel to the reference plane intersects the printed circuit board, and are formed in a shape symmetric with respect to the first intersection, and each of the pair of differential electrodes is formed in a shape symmetric with respect to a second intersection where a plane normal to the reference plane intersects the printed circuit board; wherein the dielectric liquid is filled into the closed space to the level of the first intersection; and
wherein the signal processing circuit section includes: a buffer circuit for receiving a signal of a predetermined frequency from the outside and supplying it to the common electrode; a first capacity-voltage conversion circuit for rectifying a signal taken out of a first capacitor constituted by the common electrode and one electrode of the pair of differential electrodes and converting it into voltage; a second capacity-voltage conversion circuit for rectifying a signal taken out of a second capacitor constituted by the common electrode and the other electrode of the pair of differential electrodes and converting it into voltage; and a differential amplifier circuit for producing the difference between the output voltage of the first capacity-voltage conversion circuit and that of the second capacity-voltage conversion circuit.
When a tilt sensor according to the present invention is mounted on a reference plane for measuring a tilt angle, e.g. the surface of the printed circuit board 1 becomes a the plane to be measured, the output signal of the signal processing circuit section has the following varying characteristics.
In the zero degree, or xe2x80x9cno tiltxe2x80x9d condition, the plane to be measured is not tilted in the tilt direction to be measured and only one of the pair of differential electrodes is dipped in the dielectric liquid and the output signal of the signal processing circuit section becomes a predetermined value Vn. The predetermined value Vn corresponds to the difference in capacity between the two capacitors.
When the plane to be measured is tilted in either tilt direction to be measured, the output signal of the signal processing circuit section becomes approximately the predetermined value Vn described above in the range of the tilt angle where the level of the dielectric liquid does not reach the other electrode of the pair of differential electrodes. Where the plane to be measured is tilted in either the +xcex8 or xe2x88x92xcex8 tilt directions, as described above, the predetermined value Vn is approximately obtained.
When the plane to be measured is further tilted in either tilt direction, and the level of the dielectric liquid reaches the other differential electrode of the pair of differential electrodes, variations in capacity corresponding to either differential electrode are obtained. In either of the cases where the plane to be measured is tilted in the +xcex8 or xe2x88x92xcex8 directions, a part of one differential electrode will be removed from the dielectric liquid by some degree and a part of the other differential electrode will become immersed in the dielectric liquid by a corresponding amount.
In this state, the capacity of the capacitor which has been dipped in the dielectric liquid is decreased (formed of one differential electrode and the common electrode), while the capacity of the capacitor which is dipped in the dielectric liquid is increased (formed of the other differential electrode and the common electrode).
Accordingly, the difference in capacity between the two capacitors is decreased and hence the level of the output signal from the signal processing circuit section is decreased relative to the tilt angle. Here, the output signal varies in the same way in either of the cases where the plane to be measured is tilted in the either the +xcex8 or xe2x88x92xcex8 tilt direction, as described above.
Therefore, according to the present invention, the number of threshold values required for detecting a tilt angle larger than a predetermined tilt angle is reduced to one threshold value used for either tilt direction.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.